How to block Macromedia Flash

• Uninstall Flash. But without Flash, you won't be able to access Flash-only sites or open that Thanksgiving greeting card from Aunt Sally.
• There are a number of tools that block Flash selectively (or permanently), which are nicely summarized on John Haller's Disable Flash/Uninstall Flash page.
• You can download one of many customized userContent.css files that were created to block ads, e.g. here, here or here. But you will probably have to do some maintenance. And these customized files are not foolproof: some ads get through, some good content gets blocked.